Almost all of the prior art of lens design can be subsumed under the imaging category, the purpose of which is an accurate rendering of the appearance of an object. Lenses have seen little use in the illumination field, where reflectors have predominated because of their lower cost and superior efficiency when used with conventional light sources. Thus, there has been little emphasis on the design of lenses for illumination, where the purpose is the fulfillment of a prescribed pattern of light distribution, and imaging of the light source is undesirable. Many of these prescriptions are for rectangular and other non-circular light patterns.
Because optical lenses have overwhelmingly been formed by grinding and polishing, their surfaces are figures of revolution, such as spheres, tori, and cylinders. In general, figures of revolution are not suitable for forming illumination patterns that are not circularly symmetric.
The present invention embodies a different method of lens design than that of optical imaging lenses. It utilizes shapes that are not figures of revolution, but which can be manufactured by molding of plastic or glass. They are especially suitable for use with light-emitting diodes, the tiny sizes of which allow such lenses to be small and easier to injection mold.
The most important lighting prescriptions addressed by the present invention are for vehicular lamps, by the Society of Automotive Engineers, and for ships, by the United States Coast Guard. Particular embodiments of the present invention address these prescriptions. These are far-field prescriptions for an angular distribution of light, whereas illumination prescriptions are for some nearby surface, such as the walls or ceiling of a room.
The current art of luminaire design utilizes the method of computerized searching through a number of design variations, with ray tracing used to evaluate the closeness of a candidate luminaire's output to the desired light distribution. This technique is not completely satisfactory because the vastness of the design space precludes an exact match of luminaire output to prescribed output, given that the design starting point is only a guess.
An example of traditional design is the lenslet array utilized in automotive signal lamps. Numerous small lens elements, usually spherical, cylindrical, or toric, transform the collimated beam from a reflector into a wide-angle beam shaped to fulfill government standards. Such combinations of reflector and lenslet arrays, however, typically have poor values of efficiency, such as one third. Although the reflector can be blamed for much of this inefficiency, also at fault is the restriction of lenslet shapes to spheres, cylinders, and torics (formed by rotating cutting elements), which greatly limits the designer's ability to match the shape of the output beam to the prescribed pattern. Such a match maximizes efficiency, since every point of the specification can be met with a minimum amount of light.
The general design of rotationally symmetric luminaires uses the method of matching the cumulative distribution of source intensity with that of the desired output. Cumulative intensity runs from 0 to 100%, starting at the optical axis and going outwards to the edge of the desired output pattern. Another cumulative distribution is calculated for the intensity of the light source, over the angular range to be redirected by the luminaire. Then, any angle of a ray from the source, having a particular percentage of cumulative source intensity, is redirected into an output angle having the same percentage of cumulative output intensity. From these two angles is calculated the angle the luminaire surface must have to perform the redirection. Then the actual luminaire surface is derived by radial integration outwards from an initial starting point. The resultant shape has the slope necessary to redirect the light from a rotationally symmetric source into a prescribed rotationally symmetric output pattern.
This method of matching cumulative distributions is not suitable, however, for cases where either the source intensity distribution or the desired output pattern are not figures of revolution, that is, azimuthally constant. (An example of such a source is a light-emitting diode with two bonding pads.) This is because a cumulative distribution is inherently one-dimensional, while the two dimensions of angle space prevent a unique line integral from being used to calculate a meaningful index of the shape of the distribution.
In optical lens design, the conceptually closest design method is that of anamorphic lenses. These, however, are designed for a prescribed distortion pattern, a quite different matter than fulfilling a prescribed variation in luminous intensity.